1. Field of the Invention
The invention relates to a memory element, and more particularly to a phase-change memory element and method for fabricating the same.
2. Description of the Related Art
Phase-change memory technology requires high reliability, fast speeds, low current, and low operating voltage, in order to function as a viable alternative to current memory technologies such as flash and DRAM. A phase-change memory cell must therefore provide low programming current, low operating voltage, a smaller cell size, a fast phase transformation speed, and a low cost. These requirements are difficult to meet given the current state of the art.
Current phase-change memory technology makes use of heating at the interface between a metal electrode contact and the phase-change material. More effective heating requires a smaller contact area, or equivalently a smaller heating area. A benefit of this strategy is simultaneous reduction of cell size. However, reducing the area results in higher cell resistance, which increases the required driving voltage. All other considerations being the same, the amount of Joule heating is conserved, meaning the operating voltage is inversely proportional to the programming current. This is clearly not desirable. Reducing heating area does not necessarily improve other performance features. Phase transformation speed requires good thermal uniformity within the active region of the cell. The rate of phase-change is extremely sensitive to temperature. Non-uniform heating results in a loss of reliability due to accumulation of incomplete phase-change in the programming volume.
A previously proposed approach uses the intersection between a lower metal electrode contact (width defined by a metal spacer deposition) and a phase-change material spacer (width defined by a phase-change material spacer deposition) to define a small area. However, the thin width of the phase-change spacer raises the operating voltage due to high resistance. It is not feasible to shorten the spacer sidewall height, since that would adversely affect the conformality. On the other hand, the upper metal contact can be thickened so that the contact point can be lowered, but such an enhanced heat sink lowers heating efficiency.
Therefore, it is desirable to devise a phase-change memory cell structure that addresses the aforementioned problems.